Salen Complexes Research Articles

Development and mechanistic investigation of the dehydrogenation of alcohols with an iron(III) salen catalyst.

The iron(III) salen complex (R,R)-N,N'-bis(salicylidene)-1,2-cyclohexanediamineiron(III) chloride has been developed as a catalyst for the acceptorless dehydrogenation of alcohols. The complex catalyzes the direct synthesis of imines in good yields from different primary alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with density functional theory calculations. In contrast to the corresponding manganese(III) salen-catalyzed dehydrogenation, it has not been possible to identify a homogeneous catalytic pathway with the iron complex. Instead, poisoning experiments with trimethylphosphine and mercury indicated that the catalytically active species are heterogeneous small iron particles.

Synthesis of amphiphilic chiral salen complexes and their conformational manipulation at the air-water interface.

A series of amphiphilic salen complexes, [L1a,bM] and [L2a,bM], were designed and synthesized. These complexes consist of two or four hydrophilic triethylene glycol (TEG) chains and a hydrophobic π-extended metallosalen core based on naphthalene or phenanthrene. The obtained amphiphilic complexes, [L1bM] (M = Ni, Cu, Zn), formed a monolayer at the air-water interface, while the monocationic [L1bCo(MeNH2)2](OTf) did not form a well-defined monolayer. The number of hydrophilic TEG chains also had an influence on the monolayerformation behavior; the tetra-TEG derivatives, [L1bNi] and [L2bNi], showed a pressure rise at a less compressed region than the bis-TEG derivatives, [L1aNi] and [L2aNi]. In addition, the investigation of their compressibility and compression modulus suggested that the tetra-TEG derivatives, [L1bNi] and [L2bNi], are more flexible than the corresponding bis-TEG analogues, [L1aNi] and [L2aNi], and that the phenanthrene derivatives [L1a,bNi] were more rigid than the corresponding naphthalene analogues, [L2a,bNi]. The Langmuir-Blodgett (LB) films of one of the complexes, [L1bNi], showed CD spectra slightly different from that in solution, which may originate from the unique anisotropic environment of the air-water interface. Thus, we demonstrated the possibility of controlling the chiroptical properties of metal complexes by mechanical compression.

Salen, salan and salalen zinc(II) complexes in the interaction with HS-: time-resolved fluorescence applications.

In the current work we investigate the route of interaction of a newly synthesized family of zinc complexes with HS- by a plethora of different spectroscopic techniques. A computational analysis on the time dependent density functional theory (TD-DFT) level explored the overall fluorescence properties of the title complexes and their different fluorescence responses to HS-. Time-resolved fluorescence experiments were also performed and highlight the great potential of the current systems to be implemented as HS- fluorescent sensors.

Fluorine-based Zn salan complexes.

We synthesised and characterised the racemic and chiral versions of two Zn salan fluorine-based complexes from commercially available materials. The complexes are susceptible to absorbing H2O from the atmosphere. In solution (DMSO-H2O) and at the millimolar level, experimental and theoretical studies identify that these complexes exist in a dimeric-monomeric equilibrium. We also investigated their ability to sense amines via19F NMR. In CDCl3 or d6-DMSO, strongly coordinating molecules (H2O or DMSO) are the limiting factor in using these easy-to-make complexes as chemosensory platforms since their exchange with analytes requires an extreme excess of the latter.

INVESTIGATING THE STRUCTURAL AND SURFACE PROPERTIES OF COPPER AND NICKEL COMPLEXES FLUORINE-BASED SALEN TYPE SCHIFF BASES

Two new transition metal complexes, NiL1 (1) and CuL2 (2), with formulations C18H12F6N2NiO4 and C18H12CuF6N2O4, respectively, were synthesized through the slow evaporation method in ethanol. The synthesized complexes were extensively characterized using X-ray diffraction, Fourier transform infrared spectroscopy, and UV-Vis spectroscopy techniques. X-ray crystallographic analysis revealed that the Ni(II) and Cu(II) salen complexes consist of double deprotonated tetrafunctional chelating ligands, namely L1 for NiL1 and L2 for CuL2, along with the corresponding metal cations, Ni+2 and Cu+2. The asymmetric unit of NiL1 comprises half of the molecule, while the asymmetric unit of CuL2 consists of a single molecule. In the crystal lattice, the molecules of both complexes are interconnected through C–H···O and C–H···F hydrogen bonds. Additionally, weak π···π interactions contribute to the formation of a layered structure in both complexes. To gain further insight into the intermolecular interactions, Hirshfeld surface analysis was employed, enabling the identification of atom locations with potential for hydrogen bonding and providing a quantitative assessment of these interactions in the complexes. The analysis revealed that the dominant contributions to the Hirshfeld surface originated from F···H, H···H, and O···H contacts in both NiL1 and CuL2 complexes.

NiII and CuII complexes of a salen ligand bearing ferrocenes in its secondary coordination sphere.

Herein, we report the synthesis, spectroscopic characterization and electrochemical investigation of the NiII and CuII complexes of a novel Sal ligand bearing two ferrocene moieties attached at its diimine linker, M(Sal)Fc. The electronic spectra of M(Sal)Fc are near identical to its phenyl-substituted counterpart, M(Sal)Ph, indicating the ferrocene moieties exist in the secondary coordination sphere of M(Sal)Fc. The cyclic voltammograms of M(Sal)Fc exhibit an additional two-electron wave in comparison to M(Sal)Ph, which is assigned to the sequential oxidation of the two ferrocene moieties. The chemical oxidation of M(Sal)Fc, monitored by low temperature UV-vis spectroscopy, supports the formation of a mixed valent FeIIFeIII species followed by a bis(ferrocenium) species upon sequential addition of one and two equivalents of chemical oxidant. The addition of a third equivalent of oxidant to Ni(Sal)Fc yielded intense near-IR transitions that are indicative of the formation of a fully delocalized Sal-ligand radical (Sal˙), while the same addition to Cu(Sal)Fc yielded a species that is currently under further spectroscopic investigation. These results suggest the oxidation of the ferrocene moieties of M(Sal)Fc does not affect the electronic structure of the M(Sal) core, and these are thus in the secondary coordination sphere of the overall complex.

Heterogenization of Salen Metal Molecular Catalysts in Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

AbstractIntegrating a molecular catalyst with a light harvester into a photocatalyst is an effective strategy for solar light conversion. However, it is challenging to establish a crystallized framework with well‐organized connections that favour charge separation and transfer. Herein, we report the heterogenization of a Salen metal complex molecular catalyst into a rigid covalent organic framework (COF) through covalent linkage with the light‐harvesting unit of pyrene for photocatalytic hydrogen evolution. The chemically conjugated bonds between the two units contribute to fast photogenerated electron transfer and thereby promote the proton reduction reaction. The Salen cobalt‐based COF showed the best hydrogen evolution activity (1378 μmol g−1 h−1), which is superior to the previously reported nonnoble metal based COF photocatalysts. This work provides a strategy to construct atom‐efficient photocatalysts by the heterogenization of molecular catalysts into covalent organic frameworks.

Heterogenization of Salen Metal Molecular Catalysts in Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution.

Integrating a molecular catalyst with a light harvester into a photocatalyst is an effective strategy for solar light conversion. However, it is challenging to establish a crystallized framework with well-organized connections that favour charge separation and transfer. Herein, we report the heterogenization of a Salen metal complex molecular catalyst into a rigid covalent organic framework (COF) through covalent linkage with the light-harvesting unit of pyrene for photocatalytic hydrogen evolution. The chemically conjugated bonds between the two units contribute to fast photogenerated electron transfer and thereby promote the proton reduction reaction. The Salen cobalt-based COF showed the best hydrogen evolution activity (1378 μmol g-1 h-1 ), which is superior to the previously reported nonnoble metal based COF photocatalysts. This work provides a strategy to construct atom-efficient photocatalysts by the heterogenization of molecular catalysts into covalent organic frameworks.

Concise Synthesis of Glycerophospholipids.

Glycerophospholipids are major components of cellular membranes and provide important signaling molecules. Besides shaping membrane properties, some bind to specific receptors to activate biological pathways. Untangling the roles of individual glycerophospholipids requires clearly defined molecular species, a challenge that can be best addressed through chemical synthesis. However, glycerophospholipid syntheses are often lengthy due to the contrasting polarities found within these lipids. We now report a general strategy to quickly access glycerophospholipids via opening of a phosphate triester epoxide with carboxylic acids catalyzed by Jacobsen's Co(salen) complex. We show that this method can be applied to a variety of commercially available fatty acids, photoswitchable fatty acids, and other carboxylic acids to provide the corresponding glycerophosphate derivatives.

Mechanochemical synthesis of unsymmetrical salens for the preparation of Co-salen complexes and their evaluation as catalysts for the synthesis of α-aryloxy alcohols via asymmetric phenolic kinetic resolution of terminal epoxides.

In this paper, we report the mechanochemical synthesis of unsymmetrical salens using grinding and ball milling technologies, respectively, both of which were afforded in good yield. The chelating effect of the unsymmetrical salens with zinc, copper, and cobalt was studied and the chiral Co–salen complex 2f was obtained in 98% yield. Hydrolytic kinetic resolution (HKR) of epichlorohydrin with water catalyzed by complex 2f (0.5 mol %) was explored and resulted in 98% ee, suggesting complex 2f could serve as an enantioselective catalyst for the asymmetric ring opening of terminal epoxides by phenols. A library of α-aryloxy alcohols 3 was thereafter synthesized in good yield and high ee using 2f via the phenolic KR of epichlorohydrin.

ZnII-DyIII single-molecule magnets constructed by salen-type ligand, acetate and β-diketonate

A series of ZnII-DyIII complexes with molecular formula [LZn(CH3COO)Dy(hfac)2] (1), [LZn(CH3COO)2Dy(dbm)]·CH2Cl2 (2), [LZn(CH3COO)Dy(btfa)2]·CH2Cl2 (3), (H2L = N,N'-bis(3-methoxysalicylidene)-1,2-phenylenediamine, hfac = hexafluoroacetylacetone, dbm = dibenzoylmethane, btfa = benzoyltrifluoride acetone) were structurally and magnetically characterized. They have similar ZnII-DyIII structural cores bridged by N and O atoms of the hexadentate salen type ligand (L). The main difference between them is the peripheral β-diketonate co-ligands surrounding DyIII ion. Dynamic magnetic properties measurements revealed that complexes 1-3 exhibit the field-induced SMM behavior with relaxation energy barriers Ueff/kB of 45.49 K, 69.28 K and 101.34 K, respectively. The ac magnetic susceptibility of diluted sample 3Dy0.05Y0.95 confirmed the single molecule original relaxation, in which the quantum tunneling of magnetization (QTM) was suppressed efficiently and a butterfly-shaped hysteresis loop at 1.8 K was observed.

Structural investigation of Fe(III)-salen complexes as “turn-on” fluorogenic probes for selective detection of pyrophosphate ions

In this work, we revisited the disassembly approach (also known as the Zelder's approach) recently proposed for sensing pyrophosphate (PPi) in water and based on the decomposition of metal-salen complexes. A systematic study devoted to the structural optimization of this novel class of PPi-responsive fluorogenic probes was conducted. Screening of eight different vicinal diamines (i.e., bridge of the salen ligand) combined with the use of 8-formyl-7-hydroxycoumarin (i.e., salicylaldehyde derivative) as the fluorescent reporter, has led to a set of novel and fully characterized coumarin-salen Fe(III) complexes. A series of analytical validations helped us to identify that coumarin-salen Fe(III) complexes derived from ethylenediamine and racemic 1,2-propylenediamine backbones exhibit the best and selective PPi-sensing performances (the limits of detection were estimated as 3.15 × 10−6 M and 2.81 × 10−6 M respectively). The implementation of both fluorescence time-course measurements and RP-HPLC-fluorescence analyses has enabled us to gain further insights into the activation mechanism of the disassembly-based probes. This study therefore demonstrates that the disassembly approach is a valuable strategy to achieve fluorogenic activity-based sensing of anions.

Tuning cationic transport in Nisalen polymers via pseudo-crown functionality

Polymeric films based on nickel complexes with salen-type ligands have received considerable attention recently owing to electrocatalytic, electrochromic, and charge storing properties. The latter makes them suitable metal-organic materials for electrochemical power sources, i.e., batteries and supercapacitors. Optimization of the properties of electrode materials is closely linked to the understanding of charge storage mechanisms. The introduction of CH3O substituent into the molecule results in peculiar ionic transport mechanism, owing to the possibility of alkaline ions coordination. Here we study the recharging mechanism of poly[Ni(CH3Osalen)] films in various electrolyte solutions. In presence of alkali ions, the electronic effects of methoxy substituent provide mixed anionic and cationic charge compensation mechanism, as cations reversibly coordinate to the present pseudo-crown functionality. By applying the combination of XRD, CV/EQCM and EIS methods to the film in electrolytes containing Li+, Na+, K+, and Et4N+ cations, and BF4−, ClO4− and bistrifluoromethanesulfonimidate (TFSI−) anions, we propose a model that describes the ionic transport in such polymeric films and allows to estimate the anionic and cationic contribution to the total amount of transferred species during charging and discharging.

Synthesis of anionic rare-earth metal salan complexes as catalysts for copolymerization of cyclohexene oxide with cyclic anhydrides

• New anionic rare-earth metal salan complexes [Et 3 NH][salanMCl 2 ]. • Promoting the ROCOPs without cocatalyst producing alternating polyesters. • Different substituents and central metals affect the catalytic activity. Several new rare-earth metal salan complexes of [Et 3 NH][salanMCl 2 ] were synthesized and evaluated as catalysts for copolymerization of cyclohexene oxide (CHO) with cyclic anhydrides. It was found that all of the complexes show moderate activities toward the copolymerizations without any cocatalysts, resulting in alternating polyesters with number-average molecular weight (M n ) of 1.7–2.2 kg/mol. Both the different substituents and central rare-earth metals of the catalysts affect the catalytic activity. Catalysts carrying bulkier substituents show lower activity. In general, [Et 3 NH][salanYCl 2 ] complexes show relatively high activity in comparison to their lutetium analogues bearing the same ligands.

Variation of the Emission Efficiency and Wavelength from Fluorescent Zinc Salen Complexes upon Systematic Structural Modifications.

Understanding the photophysical properties of metal salen complexes is not straightforward because the emission efficiency is altered irregularly upon structural modifications. The present study prepared zinc salen complexes with systematic structural variations to pinpoint critical factors to determine the emission efficiency. One of the important experimental observations is the regiochemistry of a phenolate substituent affecting emission efficiency from a salicylidene fluorophore, which is nicely assigned as arising from the photoexcited electronic structure of metal salen complexes. Another significant finding is the thermal fluctuation of a salen ligand arising from the mismatched ligand-metal interaction, which has a significant impact on fluorescence lifetime. The present study sheds light on hidden factors that alter photophysical properties of a metal salen complex, which provide valuable insights into designing new photoactive salen ligands.

Post-Modification of Copolymers Obtained by ATRP for an Application in Heterogeneous Asymmetric Salen Catalysis.

Copolymers are valuable supports for obtaining heterogeneous catalysts that allow their recycling and therefore substantial savings, particularly in the field of asymmetric catalysis. This contribution reports the use of two comonomers: Azido-3-propylmethacrylate (AZMA) bearing a reactive azide function was associated with 2-methoxyethyl methacrylate (MEMA), used as a spacer, for the ATRP synthesis of copolymers, and then post-functionalized with a propargyl chromium salen complex. The controlled homopolymerization of MEMA by ATRP was firstly described and proved to be more controlled in molar mass than that of AZMA for conversions up to 63%. The ATRP copolymerization of both monomers made it possible to control the molar masses and the composition, with nevertheless a slight increase in the dispersity (from 1.05 to 1.3) when the incorporation ratio of AZMA increased from 10 to 50 mol%. These copolymers were post-functionalized with chromium salen units by click chemistry and their activity was evaluated in the asymmetric ring opening of cyclohexene oxide with trimethylsilyl azide. At an equal catalytic ratio, a significant increase in enantioselectivity was obtained by using the copolymer containing the largest part of salen units, probably allowing, in this case, the more favorable bimetallic activation of both the engaged nucleophile and electrophile. Moreover, the catalytic polymer was recovered by simple filtration and re-engaged in subsequent catalytic runs, up to seven times, without loss of activity or selectivity.

Exploring the Reaction Mechanism of C-H Oxidation by Copper-Salen Complexes.

The mechanism of C-H oxidation of propylene (C3H6) and 1-phenyl-1-pentyne (C3H7-C≡C-Ph) by HOOR (R═Me, tBu) and 3O2 by a copper-salen complex was explored by computations. The most noteworthy step is the complexation of two Cu salens to the peroxide to form either the LCuOH/LCuOR pair or an OH-bridged complex LCu(μ-OH)CuL plus OR. The latter pathway involves an avoided crossing of two triplet electronic states. The LCuOH complex can abstract a hydrogen atom from C3H6 and the C3H5 radical plus 3O2 forms the complex LCuOOC3H5. Migration of a hydrogen to the proximal oxygen atom reforms LCuOH and acrolein HC(O)CH═CH2.

Efficient and Selective Chemical Recycling of CO2 -Based Alicyclic Polycarbonates via Catalytic Pyrolysis.

Chemical recycling of polymers to their constituent monomers is the foremost challenge in building a sustainable circular plastics economy. Here, we report a strategy for highly efficient depolymerization of various CO2 -based alicyclic polycarbonates to epoxide monomers in solvent-free conditions by a simple CrIII -Salen complex mediated catalytic pyrolysis process. The chemical recycling of the widely studied poly(cyclohexene carbonate) exhibits excellent reactivity (TOF up to 3000 h-1 , 0.1 mol % catalyst loading) and high epoxide monomer selectivity (>99 %). Mechanistic investigation reveals that the process proceeds in a sequential fashion via a trans-carbonate intermediate.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation.

Oxidation of a series of CrV nitride salen complexes (CrVNSalR) with different para-phenolate substituents (R = CF3, tBu, NMe2) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para-phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords CrVI nitrides ([CrVINSalR]+; R = CF3, tBu) and a localized CrV nitride phenoxyl radical for the more electron-donating NMe2 substituent ([CrVNSalNMe2]•+). The facile nitride homocoupling observed for the MnVI analogues was significantly attenuated for the CrVI complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the CrV ligand radical derivative ([CrVNSalNMe2]•+) in comparison to the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu). As a result, [CrVNSalNMe2]•+ reacts with B(C6F5)3, thus exhibiting similar nucleophilic reactivity to the neutral CrV nitride derivatives. In contrast, the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu) act as electrophiles, displaying facile reactivity with PPh3 and no reaction with B(C6F5)3. Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to CrVI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.

New Heterotrinuclear CuIILnIIICuII (Ln = Ho, Er) Compounds with the Schiff Base: Syntheses, Structural Characterization, Thermal and Magnetic Properties.

New heterotrinuclear complexes with the general formula [Cu2Ln(H2L)(HL)(NO3)2]·MeOH (Ln = Ho (1), Er (2), H4L = N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane) were synthesized using compartmental Schiff base ligand in conjugation with auxiliary ligands. The compounds were characterized by elemental analysis, ATR-FTIR spectroscopy, X-ray diffraction, TG, DSC, TG-FTIR and XRD analysis. The N2O4 salen-type ligand coordinates 3d and 4f metal centers via azomethine nitrogen and phenoxo oxygen atoms, respectively, to form heteropolynuclear complexes having CuO2Ln cores. In the crystals 1 and 2, two terminal Cu(II) ions are penta-coordinated with a distorted square-pyramidal geometry and a LnIII ion with trigonal dodecahedral geometry is coordinated by eight oxygen atoms from [CuII(H2L)(NO3)]− and [CuII(HL)(NO3)]2− units. Compounds 1 and 2 are stable at room temperature. During heating, they decompose in a similar way. In the first decomposition step, they lose solvent molecules. The exothermic decomposition of ligands is connected with emission large amounts of gaseous products e.g., water, nitric oxides, carbon dioxide, carbon monoxide. The final solid products of decomposition 1 and 2 in air are mixtures of CuO and Ho2O3/Er2O3. The measurements of magnetic susceptibilities and field dependent magnetization indicate the ferromagnetic interaction between CuII and HoIII ions 1.